This nonprovisional application claims priority under 35 U.S.C. xc2xa7119(a) on patent application No. 2002-057720 filed in JAPAN on Mar. 4, 2002, which is herein incorporated by reference.
1. Field of the Invention
The present invention relates to an exhaust emission control device for an internal combustion engine, particularly to a technology for preventing super heating melt down of a particulate filter disposed in an exhaust passage of a diesel engine.
2. Description of the Related Art
An exhaust gas from a diesel engine mounted on a vehicle such as a bus and a truck contains a large quantity of particulate matter (abbreviated as PM), in addition to HC, CO and NOx.
Thus, a technology in which a diesel particulate filter (abbreviated as DPF) for capturing the PM is disposed as a post processor of the diesel engine in an engine exhaust system, and the captured PM is heated by an external heat source, and burned and removed has been developed and put into practical use.
Recently, a continuous regeneration DFP has been developed which can continuously treat the PM on the DFP by providing an oxidation catalyst that supplies an oxidant for oxidizing and removing the PM on the upstream side of the DPF.
By the way, in the DPF, even when no external heat source is used for heating, the PM is naturally combusted, burned and removed, if the temperature of the DPF rises above a certain value. For example, when the vehicle runs up an ascending slope with the diesel engine in high load driving, the temperature of the DPF reaches the certain value, or is elevated near the certain value (slightly lower than the certain value), so that the PM is naturally combusted under high oxygen concentration or combusted with high possibility.
From this reason, when the diesel engine is suddenly switched from high load driving to idle driving, for example, when the vehicle stops immediately after running up an ascending slope, a combustion gas is rapidly decreased to reduce the flow rate of exhaust gas, and the heat taking away amount is reduced, resulting in a problem that when the PM is combusting, the DPF sharply rises in temperature due to a heat of combustion to more likely melt down due to superheating. Also, under the same conditions, the heat taking away amount is reduced and the oxygen concentration is increased even when the PM is in a state immediately before combustion, resulting in a problem that when the PM comes to combustion, the DPF also sharply rises in temperature due to a heat of combustion to more likely melt down due to superheating.
Thus, it is conceived that when the diesel engine is switched from high load driving to idle driving, the engine speed is increased irrespective of the deposit amount of PM and the temperature of the DPF, to prevent the flow rate of exhaust gas from being reduced and keep the heat taking away amount.
However, it is inefficient to increase the engine speed even in a situation where it is hardly apprehended that the DPF is superheated, for example, when the deposit amount of PM is relatively small, or when the temperature of the DPF is not too high, because there is a risk that the fuel consumption is aggravated, or the DPF is supercooled.
An object of the invention is to provide an exhaust emission control device for an internal combustion engine that can efficiently and securely prevent a particulate filter disposed in an exhaust passage of the diesel engine from melting down due to superheating.
In order to achieve the above object, a first aspect of the invention provides an exhaust emission control device for an internal combustion engine, comprising: a particulate filter for capturing the particulate matter in an exhaust gas, the particulate filter disposed in an exhaust system for the internal combustion engine; deposit amount detecting means for detecting a deposit amount correlation value of the particulate matter captured by the particulate filter; temperature detecting means for detecting a temperature correlation value of the particulate filter; idle rotating speed control means for controlling the rotating speed of the internal combustion engine to be a target idle rotating speed when the internal combustion engine is in an idle driving state; and correction means for correcting the target idle rotating speed to the higher rotating speed to prevent the particulate filter from being overheated in accordance with detection results of the deposit amount detecting means and the temperature detecting means.
That is, when the internal combustion engine is in the idle driving state, the target idle rotating speed is controlled by the idle rotating speed control means. If there is a risk that the particulate filter is overheated, the target idle rotating speed is corrected to the higher rotating speed by the correction means in accordance with the detection results of the deposit amount detecting means and the temperature detecting means.
Accordingly, when the vehicle stops immediately after running up an ascending slope, in other words, when the internal combustion engine is changed to the idle driving immediately after the high load driving, the flow rate of exhaust gas is reduced and the heat taking away amount is decreased, with the result that when the particulate matter (PM) captured by the particulate filter (DPF) is combusted due to the high load driving, the DPF sharply rises in temperature. But the target idle rotating speed is corrected to the higher rotating speed in accordance with the deposit amount correlation value of PM and the temperature correlation value of the DPF to prevent the flow rate of exhaust gas from being reduced and keep the heat taking away amount, whereby the DPF is efficiently and securely prevented from melting down to superheating. Also, under the same conditions, the heat taking away amount is decreased and the oxygen concentration is increased even when the PM is in a state immediately before combustion, with the result that when the PM comes to combustion, the DPF also sharply rises in temperature due to a heat of combustion. But if the target idle rotating speed is corrected to the higher rotating speed in accordance with the deposit amount correlation value of PM and the temperature correction value of the DPF, it is possible to appropriately prevent the flow rate of exhaust gas from being reduced, keep the heat taking away amount, and efficiently and securely prevent the DPF from melting down due to superheating.
A second aspect of the invention is characterized in that the correction means corrects the target idle rotating speed to the higher rotating speed, when the deposit amount correlation value of the particulate matter detected by the deposit amount detecting means is larger.
Accordingly, when the deposit amount correlation value of PM is larger, the quantity of combustion heat produced by combustion of PM is increased, so that the DPF is more likely superheated. However, the target idle rotating speed is corrected to the higher rotating speed, as the deposit amount correlation value of PM is larger, whereby it is possible to keep the heat taking away amount to have an appropriate flow rate of exhaust gas to prevent overheating the DPF. On the other hand, when the deposit amount correlation value of PM is not so large, the correction amount of the target idle rotating speed is suppressed to the required minimum value to prevent the fuel consumption from being aggravated and the supercooling of the DPF, whereby the DPF is efficiently and securely prevented from melting down due to superheating.
Also, a third aspect of the invention is characterized in that the correction means corrects the target idle rotating speed to the higher rotating speed when the temperature correlation value of the particulate filter detected by the temperature detecting means is above a predetermined value, or inhibits or suppresses the correction for the target idle rotating speed when the temperature correlation value is at or below the predetermined value.
Accordingly, the DPF is likely to be superheated as the temperature correlation value of the DPF is large. When the temperature correlation value of the DPF is above the predetermined value, the target idle rotating speed is corrected to a high rotating speed, whereby it is possible to keep the heat taking away amount to have an appropriate flow rate of exhaust gas to avoid overheating of the DPF. On the other hand, when the temperature correlation value of the DPF is at or below the predetermined value, the target idle rotating speed is not corrected to the higher rotating speed to prevent the fuel consumption from being aggravated and the supercooling of the DPF, whereby the DPF is efficiently and securely prevented from melting down due to superheating.
Also, a fourth aspect of the invention is characterized in that the predetermined value is set to a lower temperature, when the deposit amount correlation value of the particulate matter detected by the deposit amount detecting means is larger. Accordingly, the DPF is likely to be superheated as the deposit amount correlation value of PM and the temperature correlation value of the DPF are large, but the predetermined value as a threshold for discriminating the temperature correlation value of the DPF is decreased to a lower temperature. Thereby, the target idle rotating speed is corrected to a high rotating speed, when the temperature correlation value of the DPF is above the predetermined value in accordance with the deposit amount correlation value of PM, to keep the heat taking away amount to have an appropriate flow rate of exhaust gas. On the other hand, when the temperature correlation value of the DPF is lower than or equal to the predetermined value in accordance with the deposit amount correlation value of PM, the target idle rotating speed is not corrected to the higher rotating speed to prevent the fuel consumption from being aggravated and the supercooling of the DPF, whereby the DPF is efficiently and securely prevented from melting down due to superheating.